This relates to generation of sequential color illumination in solid state laser projection systems and the like.
Current solid state illuminated projectors producing more than about 1000 lumens utilize blue laser diodes and a spinning phosphor wheel. The illumination typically involves sequential generation of blue, green and red color light, and the sequentially generated different colored light is directed to a pixel light modulator of one type or other. The pixel light modulator may, for example, be a spatial light modulator such as a Texas Instruments DLP™ digital micromirror device (DMD) whose mirrors are individually set using pulse-width modulation (PWM) intensity grayscaling with settings synchronized to respective time segments for illumination of the mirrors by the respective sequentially generated colors.
In a typical solid state illumination (SSI) system, at least red, green and blue color illumination time segments are generated. Other color time segments (viz., secondary color, white, and black periods) are also possible. In a usual arrangement, a green color light is generated indirectly by illuminating a green color-emitting segment of the spinning phosphor wheel with light from a typically blue laser light emitting diode (LED), while red color light is generated either indirectly by illuminating a red color-emitting phosphor segment of the same wheel with the blue laser light or directly by using a separate red light LED. The blue color light is usually generated directly using the blue laser light itself. A diffuser is typically used to reduce speckle from the coherent light.
An example solid state projector that uses blue laser LEDs and a phosphor color wheel as an illumination system and a spatial light modulator for modulation of the generated illumination is described in application Ser. No. 13/652,129 filed Oct. 15, 2012, entitled “Projector Light Source and System, Including Configuration for Display of 3D Images,” the entirety of which is incorporated herein by reference. Such system utilizes blue lasers as a direct source of blue color light and utilizes the blue lasers as an indirect source of other color light by energizing other color light producing phosphors with the blue color light from the blue lasers. The overall layout for such a system is illustrated schematically in FIG. 1, which has an insert showing the combining of outputs from banks of lasers into a single diffused blue light input beam. Because the illumination system generates one output color directly from the input source light and one or more other output colors indirectly by secondary emission, projectors utilizing such illumination systems are often referred to as hybrid SSI laser projectors.
The arrangement 100 shown in FIG. 1 is typical for a solid state projector that uses blue lasers and a phosphor wheel as a sequential color illumination source. One or more banks of lasers 102 direct blue color coherent laser light onto a dichroic filter 104 which reflects the laser light via a focusing lens 106 onto a color wheel 108. The color wheel 108 includes angularly spaced segments of respective different color wavelength light emitting phosphors formed over a light reflecting surface, as well as clear laser blue color wavelength light transmitting segments. The laser light is focused to be sequentially incident on the respective different segments as the wheel 108 is rotated.
A typical phosphor color wheel 108 as used in the described solid state projector has annular arcuate region segments (viz., sectors of an annulus defined by two radii separated by an inner angle and by the inner and outer arcs they intercept) coated with different color emitting phosphors disposed at respective angularly spaced positions in a ring, circumferentially about a circular wheel. FIG. 2 illustrates an example phosphor color wheel of this type. For image display, the color wheel 108 is rotated to move the phosphor coated ring through a given angular rotation (e.g., ½, 1 or 2 revolutions) during an image frame display time (eye integration time). The laser light input beam is directed onto the wheel annulus to illuminate an area (viz., spot) relative to which the different segments pass sequentially as the wheel rotates. The wheel is typically rotated at a constant rotational speed, with the angular extent of the respective different color generating segments determined, at least in part, by the relative brightnesses of the generated illumination.
In the example wheel 108 shown in FIG. 2, the wheel is a circular wheel having a reflective aluminum front surface (surface facing the input beam) 210 and different color generating annular sectors 212, 214, 216, 218 angularly spaced circumferentially about a marginal band. The wheel 108 is mounted centrally for rotation about a shaft at an adjustably settable, constant rotational speed (viz., one revolution per image frame display time). The illustrated wheel has two instances of angularly-spaced blue 212, green 214, red 216 and yellow 218 color generating segments which are sequentially driven past the incident input beam. Although not required, the illustrated sequence is the same in each instance. The blue segment 212 comprises a slit 220 for generating blue color by passing the input beam through the slit 220 and around an optical wraparound path (e.g., comprising reflecting elements 110, 112, 114 shown in FIG. 1), back to the projection optics. The green, red and yellow segments 214, 216, 218 comprise respective annular regions coated with different color light-emitting phosphors for respectively emitting corresponding green, red and yellow color light when energized by the incident input beam.
As the wheel 108 rotates, blue laser light from the lasers 102 is reflected by one side of filter 104 (viz., blue reflective dichroic mirror) to be sequentially incident upon the slit segments 212 and the green, red and yellow light emitting phosphor segments 214, 216, 218, respectively. When wheel 108 is rotated to a position where incident light is directed at a slit segment 212, blue light passes through slit 220 and is routed by the optical wraparound path 110, 112, 114 back to the other side of filter 104 which reflects the returned blue light along an optical path for modulation by a spatial light modulator and projection of an image onto a screen or other display surface. A diffuser 116 is optionally provided in the wraparound path to reduce imaging speckle using the returned laser light. When wheel 108 is rotated to a position where incident light is directed at a phosphor covered segment 214, 216, 218, the phosphors at that segment are energized to emit the color wavelength light characteristics of the phosphors of that segment. For the wheel 108 arrangement illustrated in FIG. 2, as the wheel rotates the laser light is first incident on a green light emitting phosphor segment 214, then on a red light emitting phosphor segment 216, and then on a yellow phosphor emitting segment 218. In each case, the emitted color (G, R, Y) is directed back through the lens 106 toward the dichroic filter 104. The filter 104 passes the phosphor emitted color for modulation by the spatial light modulator and projection of the image onto the display surface. In the illustrated arrangement, although not required, the same filter 104 is used to reflect both the incident and the returned blue light, as well as to transmit the green, red and yellow phosphor emitted light; and, although also not required, the returned blue light and transmitted green, red and yellow light are directed along the same optical path for modulation and image projection.
The spatial light modulator—which may, for example, include an addressable micromirror array such as a Texas Instruments DLP™ digital micromirror device (DMD)—includes light modulating elements that can be individually controlled in synchronism with the generated light to set the illumination intensity for that color contribution to corresponding individual ones of pixels of the displayed image. For example, the modulating elements may be controlled to provide a grayscale contribution of each color for the pixels using “on”/“off” settings specified for different time slices of a pulse width modulation (PWM) cycle time, with the greater number of “on” times providing a greater intensity, and the greater number of “off” times providing a lesser intensity—the cumulation of the PWM cycles for all the colors being integrated by the eye to give a particular color point and intensity for each pixel during a given image frame display time. Control of the modulating elements in a typical micromirror array implementation will be by changing the positioning of the individual mirrors to reflect the generated light toward or away from the display surface in accordance with weighted time segment bit plane color sequences developed from color contribution intensity data received on a frame-by-frame basis from an input imaging data stream. The data for the bits of the different color bit planes are developed and loaded in correspondence with the colors, order and timing characteristics of the particular color wheel utilized, the illustrated arrangement being just one example.
When rotated at constant rotational speed, the arcuate (angular) extent of each segment 212, 214, 216, 218 determines the amount of time that the color generated by that segment will be available for modulation to produce the corresponding color intensity contribution for the various pixels of the displayed image. The relative arcuate extents are thus established, at least in part, based upon the relative maximum intensities of the segment generated colors. Thus, the blue segment 212—which generates the brightest color because it passes the input blue laser light directly for generation of the blue color illumination—has the shortest angular extent, and the green segment 214—which generates the weakest intensity light by incident laser light energization of the color producing phosphors—has the longest angular extent. The illustrative layout shown in FIG. 2, for example, provides blue, green, red and yellow color sequences using 2×28° blue laser light transmitting slit segments 212, 2×61° green light emitting phosphor segments 214, 2×51° red light emitting phosphor segments 216, and 2×40° yellow light emitting phosphor segments 218.
FIG. 3 illustrates an example color gamut for a hybrid SSI laser projector utilizing a color wheel 108 like that shown in FIG. 2. Phosphors determine the red and green color points, and laser light passing through the slit opening 220 and the laser input beam wavelength determine the blue color point. For the wheel 108 shown in FIG. 2, the opening 220 takes the form of a window with an arcuate metal strip 222 left at the wheel circumference, radially outwardly bordering the window. This strip 222 leaves the circular wheel with an unbroken outer edge that improves rotational stability and reduces audible noise generation. The laser beam (spot) is directed to completely pass through the window opening. FIG. 4 shows an enlarged side view section of one example implementation of a clear (blue) segment 212 of the color wheel of FIG. 2.
Other arrangements for generating color sequences during relative movement of a color wheel and input light beam are also possible. An example color wheel having concentric annular tracks or rings of the respective different color emitting phosphors located at different radially spaced locations is described in Patent Application Pub. No. US 2011/0211333 A1, published Sep. 1, 2011, entitled “Wavelength Conversion,” the entirety of which is incorporated herein by reference.
An example of a blue laser light source used in SSI systems is a blue laser diode, such as commercially available from Nichia, that emits light in the 445-448 nm wavelength spectral region. Such laser diodes are relatively inexpensive and efficient. However, the dominant wavelengths of such less expensive laser diodes are shorter than typically used in non-SSI illumination system, so may result in a less aesthetically pleasing purplish blue color contribution in the displayed image.
Several approaches have been suggested to modify the blue color emitted using light from the blue lasers as a direct source for blue color generation. The use of cyan phosphor in combination with blue laser light in a system using a blue light wraparound path is described in application Ser. No. 14/155,009 filed Jan. 14, 2014, entitled “Hybrid Laser Excited Phosphor Illumination and Method,” the entirety of which is incorporated herein by reference. Other approaches for combining phosphor emitted cyan light with blue lase light for blue color generation are described in application Ser. No. 14/157,269 filed Jan. 16, 2013, entitled “Phosphor Wheel Illumination Using Laser Light Reflective Region,” the entirety of which is incorporated herein by reference. The use of cyan emission in combination with diffused blue laser light moves the blue color point coordinate to a more desirable position.
An implementation of the first approach is illustrated in FIG. 5. In this approach, a transparent plate 510 (viz., glass slide) coated with a thin layer of cyan emitting phosphor is attached over color wheel 108 in alignment with each blue segment 212 to completely cover the slit opening 220. FIG. 5 shows one plate 510 applied over the opening 220 and strip 222 of one segment 212 (top segment 212 in FIG. 5) and another plate 510 being applied to and brought into alignment with another segment 212 (bottom segment 212 in FIG. 5). The phosphor layer is thin enough to enable a portion of the blue light directed from light source 102 and reflected off one side of filter 104 onto a blue segment 212 of wheel 108 (see FIG. 1) to pass through plate 510 and slit 220, return by the wraparound path 110, 112, 114, and reflect off the other side of filter 104 into the projection optics path; while simultaneously enabling another portion of the incident blue light to energize the phosphor to emit cyan light. A part of the emitted cyan light passes through slit 220 and around the wraparound path 110, 112, 114 to the other side of filter 104. For this part, those wavelength components of the emitted cyan light that fall within the reflectivity characteristics of filter 104 are reflected by the filter and pass into the projection optics path along with the returned blue laser light. Another part of the cyan light is emitted on a return path from the segment 212 directly back toward filter 104, without going around the wraparound path 110, 112, 114. For this part, wavelength components that meet the transmissivity characteristics of filter 104 are pass through the filter into the projection optics path. FIG. 6 illustrates modification of the blue color point toward a more pleasing blue color, using the combination of direct blue and phosphor emitted cyan light. This approach has several drawbacks. The use of a separate glass slide over the metal wheel adds cost and complexity, and may cause problems due to different thermal expansion rates when the wheel heats up. Also, using the separate glass slide, the thickness of the thin layer phosphor must be tightly controlled to maintain consistency for establishing the desired blue color point coordinate. Moreover, the binder used in the phosphor coating may scatter light, thereby reducing efficiency.
The second approach does not use a slit or a wraparound path for the blue light segment. It utilizes a thin layer of cyan emitting phosphor coated over a reflective surface of the color wheel, and a polarization selective dichroic filter. A portion of input blue light of a given polarization reflects off the filter, passes through a quarter wavelength plate (QWP), reflects off the surface of the color wheel, and passes back through the quarter wavelength plate to the filter. The two passes through the quarter wavelength plate change the polarization of the blue light so that the reflected light is transmitted through the filter toward the projection optics. Another portion of the input light reflected off the filter energizes the thin phosphor coating over the reflective surface to emit cyan light back toward the filter. Components of the cyan (blue-green) light transmitted by the filter pass through the filter toward the projection optics, together with the transmitted portion of the reflected blue light. This approach requires different filter characteristics, presents similar cyan phosphor thickness control and binder scattering concerns as the first approach, and has other light throughput efficiency concerns.